Magnetostrictive materials have the property that the strain produced in the material is independent of the magnetizing force polarity applied to the material. Thus, if a sine wave alternating current is applied to a coil wrapped around a bar of positive expansion coefficient magnetostrictive material, the bar will expand to its maximum length at the positive and at the negative peaks of the sine wave to thereby produce a mechanical motion which has a fundamental frequency which is twice that of the frequency of the sine wave providing the magnetizing force. Hence, a DC magnetic bias is required to produce a fixed strain on the magnetostrictive material whereby the application of a superimposed alternating magnetic field causes the magnetic material to increase or decrease its elongation in response to the alternating sine wave magnetomotive force. The magnetic bias therefore results in the magnetostrictive material, when used in an acoustic transducer, producing an acoustic output signal frequency which is the same as the input signal frequency producing the magnetomotive of force. In the absence of biasing, the transducer acoustic output signal frequency is twice the drive frequency which results in low efficiency operation of the transducer. The frequency doubling of unbiased magnetostrictive transducers and the desirability of utilizing biasing is well known to those skilled in the art.
Biasing of the magnetostrictive material is accomplished by either a direct current supply source connected to a coil surrounding the magnetostrictive material or by using permanent magnets in a flux path of which the magnetostrictive material is an element. Permanent magnets are preferred over a direct current source since the permanent magnets eliminate circuit complexity, reduce electrical losses in the winding surrounding the magnetostrictive material, and reduce the size of the wiring and electrical coupling components.
Materials such as nickel and Permalloy, which are easily biased due to their high permeability can use ceramic or Alnico permanent magnets to supply the required bias fields. However, magnetostrictive materials, such as those made of rare earth elements, have a very low permeability and are much more difficult to bias and may involve using costly magnets.
It is therefore an object of this invention to provide a magnetostrictive transducer which does not require biasing in order to produce acoustic output power at the same frequency as that at which it is driven thereby eliminating the cost, bulkiness, circuit complexity and electrical losses associated with bias circuits provided by external direct current supply source or by permanent magnets.
Another object and feature of the invention is to provide a transducer capable of providing twice the peak-to-peak output excursion than is available from a transducer using the same length of biased positive (or negative) magnetostrictive material as in prior art transducers. Biasing of the magnetostrictive material as in the prior art transducers allows a peak-to-peak excursion of the material no greater than the strain change provided by an applied magnetic field from zero to saturation magnetic field. However, by use of two materials of opposite strain coefficient as in this invention, the peak-to-peak excursion is the sum of the strain change from zero to saturation magnetic field obtained from both positive and negative strain coefficient materials. Thus, the peak-to-peak excursion of the transducer of an embodiment of this invention is twice that available from prior art transducers, each having the same length magnetostrictive material.